1. Field of the Invention
This invention relates to an original exposure device in a slit exposure scanning type copying machine or the like.
2. Description of the Prior Art
In heat roller fixation type dry copying machines or the like, it is necessary to form a non-imaged, blank space at the leading end of copy paper in order to provide a better parting property of the copy paper from heat rollers. This is accomplished, for example, by forwardly shifting the timing at which the copy paper is fed to the image formation system. However, an original to be copied is usually positioned on an original carriage glass with one end thereof stopped by an original stopping reference member and, this had led to a problem that a shadow is created at the boundary between the original and the stopping portion of the original stopping reference member and such shadow appears as a black line in the leading end portion of the copy paper which should be a blank space, thus deteriorating the quality of the copy image.
Japanese Laid-Open Utility Model Application No. 110131/1978 discloses bevelling the end of the original carriage glass keep member on the original carriage glass side and white-painting such bevelled end to prevent creation of the aforementioned black line, but this method suffers from the problems that the formation of the part into a special shape involves high cost and that a thin original is pressed against the bevelled portion so that it is difficult for the original to be stably positioned.
Further, a method of providing a white member on the underside of the original carriage glass is known from Japanese Patent Publication No. 36502/1979, but this method suffers from the following problems.
FIGS. 1A and 1B to FIGS. 2A and 2B of the accompanying drawings show examples of the prior technique of preventing creation of black line. In FIG. 1A, a sheet-like white member 2 is provided on the underside of an original carriage glass 1 and this white member 2 is made longer than the fore end portion of an original stopping reference member 3 which serves also as an original carriage glass keep member, whereby during the exposure of the end portion of an original 4, the image of the white member 2 is projected to prevent a black line from appearing on copy paper. A main mirror 6 is an elliptical cylindrical plane mirror having the center of a light source 5 as the focus or a multiplane mirror comprising a plurality of plane mirrors. The image forming light from the original is reflected by a first scanning mirror 7 toward a lens. According to this method, however, the original 4 is illuminated from only one side thereof by the light source 5 and the main mirror 6 and this leads to a problem that during copying of a thick original such as an opened dictionary, a shadow is created in the opened portion of the original and a shadow by the original carriage keep member is also created at the other end of the original carriage. If, to solve such problem, a method of providing an auxiliary mirror 8 as shown in FIG. 1B and illuminating the original 4 symmetrically from two directions is adopted, the shadow of the white member 2 will be created on the original 4 by the auxiliary light from the auxiliary mirror 8 and this will cause a band-like under-exposure in the portion of copy paper which is immediately after the blank space of the leading end portion thereof, thus causing fog in such portion of the copy paper. The size of the shadow of the white member 2 depends on the thickness of the original carriage and the angle of incidence of the light from the auxiliary mirror 8 and is a considerably large size because the original carriage is usually thick, thus adversely affecting the original exposure past the boundary portion between the white member 2 and the original.
FIGS. 2A and 2B show improvements in the method of illuminating the original from two sides. As shown in FIG. 2A, during the exposure of the boundary portion between the original stopping reference member 3' and the end of the original 4, the image of the white member 2' is projected as in the case of FIG. 1B. Also, as shown in FIG. 2B, during the exposure of the end portion of the original past the boundary portion, the light beam from the auxiliary mirror 8 is intercepted by the white member 2', but in compensation therefor, the original 4 is illuminated by a fixed mirror 8' provided on the upper portion of the white member 2', thereby minimizing the fog phenomenon which results from a deficient amount of exposure. According to this method, however, during the slit exposure scanning, the light beam reflected by the auxiliary mirror 8 toward the slit area is greatly intercepted by the white member 2', and the central light ray reflected by the fixed mirror 8' toward the slit area for compensating for this varies its angle of incidence from moment to moment and therefore, in addition to the reflection factor and angle of the fixed mirror 8' itself, it is difficult to set the white member so as to prevent an excessive amount of exposure or a deficient amount of exposure in the end portion of the original.